An amide such as acrylamide is industrially produced by sulfuric-acid hydrolysis method which includes a step of heating a nitrile such as acrylonitrile with sulfuric acid and water to obtain an amidosulfate, a method of hydrating a nitrile in the presence of a catalyst such as copper metal, a copper oxide or a copper salt to obtain a corresponding amide, a method of hydrating a nitrile using a microbial enzyme (nitrile hydratase) to obtain an amide, a method of producing methacrylamide from acetocyanohydrin and so on.
Since an amide is extremely polymerizable, polymerization of an amide can take place in each step of any of the production methods described above or in preservation or deposition of an amide product in a crystalline state or as an aqueous solution. The polymerization can also cause problems such as deterioration in quality, decrease in yield from the production process, obstruction to liquid flow in apparatuses or tubing, and decrease in conduction of heat.
Therefore many kinds of compounds are proposed as a polymerization retarder for an amide. For example, JP-B-39-10109 discloses a method to inhibit polymerization of an amide by adding one or more compounds selected from the group consisting of thiourea, ammonium rhodanide, nitrobenzene, o-tolidine, phenothiazine, and nitroso R salt to the amide so that they coexist with the amide. Further JP-B-2548051 discloses a method for stabilizing an acrylamide solution using water-soluble monocarboxylic acid salts having at least two carbon atoms. (The term “JP-B” as used herein means an “examined Japanese patent publication”.)
However, the polymerization inhibition effects of the known methods are not sufficient, and further improvements have been demanded.